Well in an attempt to make this more balanced would be to up the percentage of units converted. If the avatar appearance would say convert 50% of the worlds units, and place them across the world. This wouldn't nerf the avatar because as you would expect, most of a civs units would become barbarians.
I think that an average strength size would need to be decided because some stacks could be too strong.
This would be a good idea, maybe some cities from each player could turn barb aswell
Although it would have to be tested to ensure the AI doesnt get affected much more than any players.